C H A P T E R 4 2
Introduction to the cardiovascular system
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in a pattern resembling a crown, which is why they are
called “coronary” arteries.
Cardiac:
Myocardial blood flow
The left coronary artery arises from the left side
of the aorta and bifurcates, or divides, into two large
vessels called the left circumflex artery (which travels
down the left side of the heart and feeds most of the
left ventricle) and the left anterior descending coronary
artery (which travels down the front of the heart and
feeds the septum and anterior areas, including much
of the conduction system). The artery arising from the
right side of the aorta, called the right coronary artery,
supplies most of the right side of the heart, including the
SA node.
The coronary arteries receive blood during diastole,
when the muscle is at rest and relaxed so that blood can
flow freely into the muscle. When the ventricle contracts,
it forces the aortic valve open, which in turn causes
the leaflets of the valve to cover the openings of the
coronary arteries. When the ventricles relax, the blood
is no longer pumped forwards and starts to flow back
towards the ventricle. The blood flowing down the sides
of the aorta closes the aortic valve and fills the coronary
arteries. The pressure that fills the coronary arteries is
the difference between the systolic (ejection) pressure
and the diastolic (resting) pressure. This is called the
pulse pressure
(systolic minus diastolic blood pressure
readings). The pulse pressure is monitored clinically to
evaluate the filling pressure of the coronary arteries.
The oxygenated blood that is fed into the heart by the
coronary circulation reaches every cardiac muscle fibre
as the vessels divide and subdivide throughout the myo
cardium (Figure 42.8).
The heart has a pattern of circulation called end-
artery circulation. The arteries go into the muscle and
end without a great deal of backup or collateral circula
tion. Normally this is an efficient system and is able to
meet the needs of the heart muscle. The heart’s supply
of and demand for oxygen are met by changes in the
delivery of oxygen through the coronary artery system.
Problems can arise, however, when an imbalance
develops between the supply of oxygen delivered to the
heart muscle and the myocardial demand for oxygen.
The main forces that determine the heart’s use of
oxygen or oxygen consumption include the following:
•
Heart rate
: The more the heart has to pump, the more
oxygen it requires.
•
Preload (amount of blood that is brought back to the
heart to be pumped throughout the body)
: The more
blood that is returned to the heart, the harder it will
have to work to pump the blood around. The volume
of blood in the system is a determinant of preload.
•
Afterload (resistance against which the heart has to
beat)
: The higher the resistance in the system, the
harder the heart will have to contract to force open
the valves and pump the blood along. Blood pressure
is a measure of afterload.
•
Stretch on the ventricles
: If the ventricular muscle
is stretched before it is stimulated to contract, more
actomyosin bridges will be formed, which will take
more energy; alternatively, if the muscle is stimulated
to contract harder than usual (which happens with
sympathetic stimulation), more bridges will be
formed, which also will require more energy.
The muscle can be stretched, as in ventricular hyper
trophy related to chronic hypertension or cardiac muscle
damage, or in heart failure when the ventricle does not
empty completely and blood backs up in the system.
The supply of blood to the myocardium can be
altered if the heart fails to pump effectively and cannot
deliver blood to the coronary arteries. This happens in
Right
auricle
Right
atrium
Left auricle
Circumflex branch
of left coronary
artery
Left circumflex
branch
Coronary sinus
Left pulmonary veins
Anterior
descending
branch of
left coronary
artery
Right
coronary
artery
Right
ventricle
Left ventricle
A
B
Superior
vena
cava
Superior
vena cava
Aortic arch
Right
ventricle
Posterior descending
branch of right
coronary artery
Inferior
vena
cava
Right pulmonary veins
Aortic valve
FIGURE 42.8
Coronary arteries and
veins.
A.
Anterior view.
B.
Posterior
view.
